In order to suspend a lighting apparatus from a ceiling, a single suspension cord unitizing a suspension string (a reinforced member) and a power supply cable is employed. In such a case, since the suspension cord serves as power supplying to the lighting apparatus as well as supporting the weight thereof, it is easily installed. And, such the suspension cord has a simple good appearance.
An exemplary suspending cord for an electrical apparatus is disclosed in Japanese Patent Publication No. H08-129905, in which a power supply cord comprising a grounded conductor cable and a voltage-applied conductor cable extending in parallel is covered with an insulating layer such as resin and fabric at the outer surface thereof. In order to supply electrical power enough for operating the lighting apparatus, such the conductor cables are made by using a stranded wire of a high conductive material such as copper. However, since such a conductor cable has small tensile strength and therefore cannot support a load of several kilograms or more, it enables suspending only a lightweight apparatus such as a household lighting. In addition, since the cord has a large diameter of about 5 mm or more, it is undesirable for suspending an elaborately designed lighting apparatus.
Another suspension wire capable of serving as a power supply wire as well as a signal wire is disclosed in Japanese Patent Publication No. H11-113702. The power supply wire serves as suspending an article, such as a picture and the like, as well as detecting a theft of the article. The wire is composed of a conductive core wire, an insulating layer being covered the outer surface of the conductive core wire and a conductive outer layer being covered the outer surface of the insulating layer. The conductive core wire and the conductive outer layer are made of a stranded wire having high tension. The conductive core wire and the conductive outer layer are electrically conducted at the distal ends thereof and connected to each of signal input terminals at the proximal ends so that the conductive core wire and the conductive outer layer can form a closed electrical circuit, which is live with weak electricity. The article such as a picture is securely held on the intermediate portion of the wire by using an additional grip.
When the wire is cut off at somewhere thereof so as to detach the picture therefrom, a conduction between the conductive core wire and the conductive outer layer will be shut off, resulting in opening the circuit having an alarm unit. When the circuit is opened, the alarm unit announces by sound or display that the wire is cut off.
The wire is able to suspend a comparatively heavy article such as a picture. However, a current which flows through the circuit is too weak to operate an electrical apparatus such as a lighting apparatus.
In general, when a current flows through a conductor cable, a dielectric current will be generated at a conduit of the conductor cable, resulting in heat generation. However, in a case of a power supply wire in which a bundle of a grounded conductor cable and a voltage-applied conductor cable extending in parallel is covered with an insulating layer such as resin and fabric at the outer surface thereof, currents of each conductor cable flows in opposite directions and induction fields are canceled each other, whereby the conduit does not generate heat. And, in a case of a power supply wire having a double ring structure comprising a conductive core wire and a conductive outer layer, which works as a grounded conductor cable and a voltage-applied conductor cable, an induction current generated by powering the grounded conductor cancels an induction current generated by powering the voltage-applied conductor cable. In addition, when the conduit is a nonconductor, an induction current is hardly generated.
In order to hold a lighting apparatus to such the power supply wire, a wire grip is employed. At this time, the wire grip is gripped at the intermediate portion of the wire and the lighting apparatus is securely held to the wire grip. Then, a terminal of the wire is connected to a terminal contact of the lighting apparatus in a manner such that a conductive core wire and the conductive outer layer are conducted to the terminal contacts of the lighting apparatus.
By the way, in such the power supply wire, a conductive outer layer may be made of a braided metal wire. The braided wire is made such that one of bundle of metal thin lines and another one of bundle of metal thin lines are braided alternatively. Such the braided wire has flexibility and contractility in the length direction. However, when the braided wire is cut off, it will loosen into the metal thin lines from the cut ends. In order to treat the cut end, first, the outer layer is slid (is ridden up) in the length direction of the wire so as to expose an insulating layer. Then, after the slid outer layer is fixed to the insulating layer by an insulating tape and the like, the insulating layer is stripped by using a nipper so as to expose a conductive core wire.
However, when the wire would be inserted through a wire-insertion bore of a wire grip with the treated end thereof in the lead, the wire would be twisted or the outer layer would become knobbed due to flexibility of the wire, resulting in that it is impossible to insert the wire through the wire-insertion bore with the end thereof in the lead. Accordingly, it is necessary to put a cap on the end. Putting the cap on the wire end requires a special tool, causing a lot of trouble. Accordingly, the braided wire with the both ends being treated in the aforesaid manner, having various predetermined lengths, must be employed. Accordingly, it is difficult to change the length of the wire when it is installed.
Japanese Utility Model Patent No. S64-47256 discloses a wire grip used for securely holding a shelf at the intermediate portion of a suspended wire. The wire grip has a slotted groove in the length direction thereof, in which the wire is inserted from the side surface of the wire grip and is securely gripped by the wire grip at the intermediate portion of the wire. Such type of the gripper is applicable to a power supply wire in which an outer layer is made of a flexible braided wire, however, it is necessary to insert the wire into the slotted groove while giving a tension to the wire. At this time, in a case of the outer layer made of a metal braided wire, even if the wire would be tensioned, the outer layer is slid with respect to the wire, whereby insertion of the wire is difficult.
In recent years, in response to popularization of a thin type liquid crystal television and utilization of a lighting apparatus for an elaborately designed structure or a display room of a shop, a suspension unit capable of suspending an electrical apparatus is on demand. To such suspension unit, sufficient strength and electrical safety are required.